1. Field of the Invention
The present invention relates to an image processing method and an image recording apparatus, and more particularly, to image processing technology for an image recording apparatus which forms an image on a recording medium by using a recording head formed with a plurality of recording elements (nozzles) arranged two-dimensionally at high density.
2. Description of the Related Art
In an inkjet recording apparatus, there is a problem in that streak-shaped density non-uniformities (hereinafter, simply called “non-uniformities”) occur due to variations in the recording characteristics of the recording elements provided in the head (for example, the ejection characteristics of the nozzles which eject liquid droplets), and technologies for correcting the ejection characteristics of nozzles by image processing have been proposed.
Here, one example of technology for correcting the ejection characteristics of nozzles in image processing according to the related art is described below with respect to FIGS. 26 to 32. In FIGS. 26 to 32, parts which are similar to or the same as those in FIGS. 1 to 25 are labeled with the same reference numerals.
FIG. 26 is a diagram showing image processing according to the related art. As shown in FIG. 26, when image data 1 of 256 tonal graduations is received, a line correction processing unit 14 multiplies the image data 1 by correction coefficients 12 (line correction coefficients di) based on the previously established ejection characteristics of the nozzles, thereby corrected image data with generating 256 tonal graduations (the object image in FIG. 1) 16.
The line correction coefficients di used in the line correction coefficient processing unit 14 (where i is the nozzle number of the nozzle 522-i provided in the head 520 shown in FIG. 29, and i=1 to 8), are generated for the respective nozzles by a line correction coefficient generation unit 2, by means of various steps, such as printing a test pattern, measuring the test pattern, and creating nozzle density correction coefficients. The correction coefficients thus generated are stored in a prescribed storage unit. The corrected image data 16 created by correction based on the line correction coefficients di generated in this way is then subjected to halftoning (for example, quantization based on an error diffusion method) in a binarization processing unit (halftoning unit) 20, thereby creating binary (multiple-value) output data (print data) 22.
FIG. 27 is a block diagram of the image processing block which is explained with reference to FIG. 26 and in which the image processing is executed. In general, a so-called error diffusion method is suitable for use in generating a binary (multiple-value) image of high quality and good reproducibility with respect to an input image. In such an error diffusion method, binarization processing is carried out in the binarization processing direction indicated by the arrowed line in FIG. 27, for each pixel in the corrected image (object image) 16 which has been subjected to the line correction processing.
In other words, in the binarization processing unit 20, the pixel values of the processed pixels (for example, values in the range of 0 to 255 in the 8-bit data) are compared with the threshold values generated by the threshold value generation unit 40, and binary output data 22 (0/255) is derived on the basis of these comparison results.
Furthermore, in the error calculation unit 24, the error between the data of the object image 16 and the output data 22 is calculated. In the error diffusion processing unit 28, correction data for an error diffusion process of the error determined by the error calculation unit 24 is calculated by means of an error diffusion filter 26. In the error diffusion process, the error in the pixel under processing is diffused into the peripheral pixels. The correction data is stored temporarily in the error buffer 30.
In the input value correction unit 32, the input data I of the peripheral pixels of the pixel under processing is corrected on the basis of the correction data stored in the error buffer 30, and is rewritten as input data I′ after the error diffusion processing. The binarization processing unit 20 then carries out binarization processing on the input data I′ after the error diffusion processing, which has been rewritten in the way as mentioned above.
FIG. 28 shows one example of an error diffusion filter 26. Of the unprocessed pixels, the error diffusion filter 26 shown in FIG. 28 diffuses 7/16 of the error into the adjacent pixel 26-2 on the right-side of the pixel under processing 26-1, and it diffuses 3/16 of the error into the lower left-side pixel 26-3, 5/16 of the error into the pixel 26-4 directly below, and 1/16 of the error into the lower right-side pixel 26-5. The binarization processing direction of the error diffusion filter 26 shown in FIG. 28 is the left to right direction in FIG. 28 (the direction indicated by the arrowed line).
Furthermore, the error diffusion processing described above is represented as follows:E(i, j)=O(i, j)—I(i, j),  (1)I′(i, j)=I(i, j)−I_cor(i, j),  (2)I_cor(i, j)=ΣM(k, l)·E(i−k, j−1),  (3)
where I is the input data, O is the output data, E is the binarization error, I′ is the corrected input data, and M is the error diffusion filter.
Japanese Patent Application Publication No. 2004-230672 discloses an image processing method which corrects density non-uniformities caused by errors in the ink volume ejected from nozzles, and Japanese Patent Application Publication No. 2004-58282 discloses an image processing method which corrects density non-uniformities caused by depositing position errors of the ink ejected from the nozzles.
According to the related art technology described above, there is a possibility that, if quantization processing is carried out on corrected image data which has undergone correction (density non-uniformity correction) processing in respect of the nozzle ejection characteristics by using an error diffusion method, then the correction of density non-uniformities is insufficient and non-uniformities can be visually perceived in the recorded image.
More specifically, as shown in FIG. 29, if binarization processing is carried out by the binarization processing unit 20 using error diffusion processing, with respect to an object image 16 (not shown in FIG. 29; see FIG. 26) obtained by multiplying the data of an input image 1 (see FIG. 26) by the line correction coefficient di, then some degree of error occurs between the line correction coefficient di and the print duty ratio 12′ (ei; where i is the number of the nozzle 4-i provided in the head 3, and i=1 to 8). The print duty ratio ei represents the ratio of the region in which dots are actually disposed with respect to the region in which dots can be formed, for each nozzle 4-i. From the viewpoint of correcting non-uniformities, it is desirable that the print duty ratio ei and the line correction coefficient di should be equal.
FIG. 30 shows a graph indicating the correlation between the line correction coefficient di and the print duty ratio ei. This graph is created by actually preparing line correction coefficients di for a head having 512 nozzles, calculating the print duty ratio ei after binarization processing using an error diffusion method, and then plotting marks representing the correlation between the two values, di and ei. In order to achieve desirable correction of density non-uniformities, preferably, the value of the line correction coefficient di and the value of the print duty ei are substantially the same (situated on the straight line 5 shown in FIG. 30); however, in actual practice, an error arises between the two values.
In response to these problems, Japanese Patent Application Publication No. 6-225124 discloses an error diffusion method having the characteristic of maintaining the line correction coefficient di in consequence. More specifically, the error is diffused only in a one-dimensional direction (binarization processing direction) by using a diffusion processing filter 6 as shown in FIG. 31. Hence, the error in the pixel 6-1 under processing is diffused only into the unprocessed pixel in the horizontal direction (in FIG. 31, the adjacent pixel 6-2 on the right-side of the pixel 6-1 under processing), and the error is not diffused into the lower left-side pixel 6-3, the pixel 6-4 directly below, and the lower right-side pixel 6-5. In other words, the error occurring during binarization processing of the image data corresponding to a particular nozzle 4-i is not diffused into the image data corresponding to the other nozzles, and therefore the line correction coefficient di is conserved as the print duty ratio ei.
However, according to the image processing method described in Japanese Patent Application Publication No. 6-225124, error is not diffused into the peripheral pixels in the direction perpendicular to the binarization processing direction (namely, the conveyance direction of the recording medium). In other words, there is no function for compensating for the dispersion of the dots in the direction parallel to the relative conveyance direction of the recording medium, and therefore dots are disposed in a continuous fashion in the vertical direction and artifacts (unevenness) 7 having a longitudinal striped shape following the vertical direction arise as shown in FIG. 32.
Japanese Patent Application Publication No. 6-225124 discloses a method for suppressing artifacts 7 occurring in the image 8 shown in FIG. 32, by adjusting the threshold values in the vertical direction used in binarization processing so as to alter the sequence of the formation of dots with respect to each line in the horizontal direction. According to this method described in Japanese Patent Application Publication No. 6-225124, the artifacts shown in FIG. 32 are reduced; however, it has been confirmed by experimentation carried out by the inventor who contrives the present invention that unsightly artifacts do occur, depending on the tonal graduations in density.